This invention relates to a drilling method.
When drilling bores in earth formations, for example to access a subsurface hydrocarbon reservoir, the drilled bore will often include sections which deviate from the vertical plane; this allows a wide area to be accessed from a single surface site, such as a drilling platform. The drilling of such bores, known as directional drilling, utilises a number of tools, devices and techniques to control the direction in which the bore is drilled. The azimuth and inclination of a bore is determined by a number of techniques, primarily through the use of measurement-while-drilling (MWD) technology, most commonly in the form of an electromechanical device located in the bottomhole assembly (BHA). MWD devices often transmit data to the surface using mud-pulse telemetry. This involves the production of pressure pulses in the drilling fluid being pumped from surface to the drill bit, a feature of the pulses, such as the pulse frequency or amplitude, being dependent on a measured parameter, for example the inclination of the bore. Currently, three main mud-pulse telemetry systems are available: positive-pulse, negative-pulse, and continuous-wave systems. By analysing or decoding the pressure pulses at surface it is possible for an operator to determine the relevant measured bore parameter.
It is among the objectives of embodiments of the present invention to utilise the pressure pulses produced by MWD apparatus for uses in addition to data transfer.
According to one aspect of the present invention there is provided a drilling method comprising:
producing pressure pulses in drilling fluid using measurement-while-drilling (MWD) apparatus; and
allowing the pressure pulses to act upon a pressure responsive device to create an impulse force on a portion of the drill string.
The impulse force resulting may be utilised in a variety of ways, including providing a hammer-drilling effect at the drill bit, and vibrating the BHA to reduce friction between the BHA and the bore wall.
The invention also relates to apparatus for implementing the method.
The pressure pulses produced by conventional MWD apparatus are typically up to around 500 psi. At this pressure it may be possible to produce a useful impulse force, however it is preferred that the pressure pulses are in the region of 700-1000 psi. Pressure pulses of this magnitude may be produced by modifying or varying the valving arrangements provided in conventional MWD apparatus, for example by modifying the valving arrangement such that the valve remains closed for a longer period. The greater magnitude of the pressure pulses will also facilitate detection at surface, particularly in situations where there may be relatively high levels of attenuation of the pulses, for example in extended reach bores or in under-balance drilling operations where the drilling fluid column may be aerated. The pressure pulses may be of any appropriate form, including positive pulses, negative pulses, and continuous waves of pulses, as are familiar to those of skill in the art.
The pressure responsive tool may be in the form of a shock tool, typically a tool forming part of a drill string which tends to axially extend or retract in response to changes in internal fluid pressure. The shock tool may be tubular and formed of two telescoping parts, with a spring located therebetween. One of the parts may define a piston, such that a rise in drilling fluid pressure within the tool tends to separate the parts and thus axially extend the tool.
The pressure responsive tool may be located above or below the MWD apparatus, and most preferably is above the MWD apparatus. The optimum location may be determined by the mud-pulse telemetry system being utilised.